There are various Converters that change one magnitude DC voltage to another. Conventional converters such as forward and flyback converters are Well described as the prior art Many text's such as George Chryssis "High-Frequency Switching Power Supplies: Theory and Design", McGraw Hill Book Co., can explain the operation of converters such as these.
In the field of zero current, zero voltage resonant converters the common goal for the designer is low material cost, high efficiency and high packing density. Current state of the art converters in this field have raised the packing density from 2 to 4 watts per cubic inch in the last few years. Unfortunately, in the battle over ever-increasing power technologies, the current resonant topologies have stopped moving the densities higher due to a number of nagging problems associated with the implementation of the resonant technologies.
In the prior art converter taught in U.S. Pat. No. 5,177,675 Archer, the topology goals were focused on zero voltage, zero current switching at medium frequencies. The series parallel approach to resonant design was there utilized to achieve a reasonably low cost, off-line, high density switching power supply for use in computer, industrial, and consumer-type products. Packing densities on the order of 4 watts per cubic inch were achieved with this topology in the last couple of years. During the implementation of U.S. Pat. No. 5,177,675, two nagging problems faced the designers in their quest to further reduce the cost, and increase the density to meet customer demands in the 1990's. The operating efficiencies of series-parallel resonant converters, such as the aforementioned patent, which utilize a reduced regulation frequency band, typically 2 to 1 or 4 to 1 for full regulation, have topped out at approximately 85% efficiency for low voltage outputs. Also, since in U.S. Pat. No. 5,177,675 there were two magnetic elements utilized, both of which cross the load line boundary of the power supply, the material and manufacturing costs of the product have limited its ability to replace older cost effective topologies in applications where the size and weight are not of paramount importance.
The major difficulties in increasing the packaging density over their current 4 watts per cubic inch lie in the magnetizing current of the transformer. As discussed in U.S. Pat. No. 5,177,675, magnetizing current is critical to successful zero voltage switching. The ramifications of changing magnetizing current are two-fold.
One major area affected is EMI (electro magnetic interference). In series-parallel converters in which the frequency range has been reduced to its current 2 to 1 or 4 to 1 range, the high voltage dV/dt on the primary switches is limited to a slope change of approximately three to one. The slope of the voltage on the high current switches in high voltage converters is one of the major sources of electromagnetic noise, particularly common mode electromagnetic noise. With the frequency shifts of current technology resonant converters at 2 to 1, and primary slope changes of three to one, the input electromagnetic filtering on the power supplies has been reduced approximately in half over previous conventional converters, or even second generation resonant converts. The goal of the resonant designers is to remove the input EMI filtering completely, however. In an age when the worldwide regulatory agencies continue to increase the EMI requirements for conducted and radiated noise, this task is becoming increasingly difficult. With the current generation of series-parallel resonant converters, under the best case operating condition, where the reset voltage on the primary switches reaches zero just before the control circuit turns on the switch, the EMI performance is excellent. This is due to the fact that the common mode current flowing between the primary and secondary through the transformer, or transformers in the case of U.S. Pat. No. 5,177,675, is low, due to the low dv/dt applied to the transformer primary winding. Under less than ideal conditions, however, the dv/dt can increase, so that the delay to switch turn-on after the voltage has reached zero can be as long as ten percent of the switching frequency. As the slope increases due to frequency shift, the common mode currents increase, causing the input filter size to grow larger than the ideal filter would be, in order to meet conducted EMI levels. Aggravating this phenomenon is the fact that since the zero voltage performance of the series parallel converters is dependent on the total capacitance reflected across the primary windings, it is necessary in practice to compromise the design of the primary switching transformer so that the leakage inductance between primary windings is higher than what the designer would normally like to have in order to achieve a lower primary capacitance. Under the ideal condition of maximum operating frequency, this works out well. However, as the control circuit is forced to reduce the operating frequency in response to changing load and line conditions, this higher leakage inductance causes ringing on the high voltage switches. This type of ringing is much lower than that associated with conventional-type converters, or first and second generation resonant converters, but given the goal of ever smaller input filters, any ringing, other than the fundamental switching frequency and its harmonics, is cause for concern.
The second problem is the decrease in efficiency on the current generation resonant converters as exemplified in U.S. Pat. No. 5,177,675 when the converter is in a condition where the frequency shift is approaching its minimum point (maximum magnetizing current). This is caused by the fact that the RMS current flowing in the switches and primary windings may be as high as 30 percent magnetizing current. This is much better than the first or second generation resonant converters of U.S. Pat. No. 4,415,959 Vincerelli or Japanese patent 1,503,925 Matshushita, but is the limiting factor in efficiency for current generation converters. This phenomenon is caused entirely by frequency shift, since the magnetizing current is a function of the operating frequency of the converter.